Getting : TypeError: undefined is not a function (evaluating 'filePath.replace('file://', '')') while uploading a blob to firebase storage

I am developing an small React Native POC with firebase storage in which a user selects image and uploads it to firebase storage under /uid/ directory. I am using react-native-firebase for using firebase in the application.

I have the uri of the image with which i am creating its blob using RNFetchBlob and then trying to upload the blob to firebase.

Here is the code for handling the blob making and uploading.

handleUploadTask = (mime = 'application/octet-stream') => {
    const {imageSource} = this.state;
    this.setState({uploading: true});

    const Blob = RNFetchBlob.polyfill.Blob;
    const fs = RNFetchBlob.fs;

    fs.readFile(imageSource.uri, 'base64')
        .then((data) => {
            return Blob.build(data, { type: `${mime};BASE64` })
        }).then(blob => {
            console.log('going to upload');
            this.uploadBlobToFirebase(blob);
        }).catch(error => {
            console.error('fs error:', error);
        })
}

and here is the uploadBlobToFirebase

uploadBlobToFirebase = (blob) => {
    const currentUserId = firebase.auth().currentUser.uid;
    const path = `/${currentUserId}/`;
    const unsubscribe = firebase.storage().ref(path).putFile(blob).on(
        firebase.storage.TaskEvent.STATE_CHANGED,
        (snapshot) => {
        const  progress = (snapshot.bytesTransferred / snapshot.totalBytes) * 100;
        //restricting progress to be shown with only 0,2,4,... 98, 100.
        if (progress % 2 == 0) {
            this.setState({
                uploadProgress: progress
            })
        }

        if (snapshot.state === firebase.storage.TaskState.SUCCESS) {
            this.setState({
                uploading: false,
            }, () => {
                //TODO: should also add this image to flatlist.
                //Get download url and set to image in flatlist.
                Alert.alert('Upload completed');
            })

          }
        },
        (error) => {
            this.setState({
                uploading: false,
            }, ()=>{
                unsubscribe();
                console.error(error);
            })
        },
      ); 
}

The function handleUploadTask is called on an onPress of a upload button and runs fine till console.log('going to upload') and then ends up into the catch with error:

'fs error:', { [TypeError: undefined is not a function (evaluating 'filePath.replace('file://', '')')] line: 115903, column: 41, sourceURL: 'http://10.0.2.2:8081/index.delta?platform=android&dev=true&minify=false' }

I am totally blank here. Not able to find the file.replace which is causing issue and why. Any help would be appreciated. Thanks.